Most of the middle repetitive DNA in Drosophila melanogaster, roughly one sixth of the genome, is composed of families of 10 to 100 repeated elements that are found at scattered locations in the chromosome arms and occupy new chromosomal positions as populations of D. melanogaster diverge. The number of repeating elements in each family seems to be maintained notwithstanding this variability of position (Young, Proc. Nat. Acad. Sci. USA, 76;6274). One of these dispersed repeated sequences, copia, is being studied as a model for the organization and function of middle repetitive DNA. Copia consists of a sequence of about 5 kb (kilobases) that is repeated 20 to 30 times in the Drosophila genome and codes for at least 4 size classes of abundantly expressed poly A plus RNA in cultured cells (Finnegan, Rubin, Young and Hogness, Cold Spring Harbor Symp. Quart. Biol. 42;1053-1063; Schwartz and Young, unpublished). The following questions will be addressed: (1) What is the transcriptional activity of dispersed middle repetitive DNA? (2) What is the relationship of extrachromosomal middle repetitive DNA to chromosomal, dispersed repeated DNA? (3) How many chromosomal arrangements are there for middle repetitive DNA? We will also initiate a study of cloned segments of Drosophila DNA derived from the Notch locus and adjacent regions of Bridges (1935) 3C interval of the X chromosome.